1. Field of the Invention
The present invention relates to a controlling device for controlling the rotational speed of an engine of a hydraulic working machine for use in civil and construction engineering, such as crane, hydraulic excavator.
2. Discussion of the Related Art
A hydraulic working machine of this type includes a hydraulic pump driven by an engine, a plurality of actuators such as hydraulic cylinders and hydraulic motors operated by oil supplied from the hydraulic pumps under pressure, and a plurality of control levers for controlling the plurality of actuators, respectively. The hydraulic pump feeds oil to each actuator under pressure via a directional change valve driven by a control lever to actuate the actuator to perform working operation. In general, the hydraulic working machine is provided with a throttle lever for setting a rotational speed of the engine to enable the operator to properly change the amount of oil fed from the hydraulic pump in accordance with operation modes.
Japanese Patent Publication No.60-38561 discloses a device for suppressing the fuel consumption for engine during the suspension period of working operation of a hydraulic working machine. This device includes a switch to enable the operator to selectively control the engine to run either at a predetermined low rotational speed for reducing the fuel consumption or at a speed designated by the throttle lever. When all the control levers are shifted to their respective neutral positions in the state that the low rotational speed mode is selected by the switch, the engine is controlled to run at the low rotational speed.
Further, the device of the above publication does not permit the engine to run at the low rotational speed as far as a predetermined time does not elapse after all the control levers are shifted to their respective neutral positions to prevent an undesirable performance that the engine is unintentionally controlled to run at the low rotational speed immediately after all the control levers are shifted to their respective neutral positions in the state that the low rotational speed mode is selected by the switch.
However, the device of the above publication has the inconvenience that to control the engine to run at the low rotational speed in the operation suspension period, the operator is required to manipulate the switch. In addition, if the operator forgets to manipulate the switch when suspending the operation, the engine continues to run at a high rotational speed designated by the throttle lever. This causes unnecessary consumption of fuel.
Furthermore, the device of the above publication immediately restores the rotational speed of the engine to a speed designated by the throttle lever when at least one of the control levers is shifted from its neutral position to its working position in the state that the engine is controlled to run at the low rotational speed. For this reason, there is a likelihood that if a high rotation speed is designated by the throttle lever, even a small shift of a control lever causes rapid increase in the rotational speed of the engine, and results in rapid increase in the amount of oil fed from the hydraulic pump and a sudden change in the actuating speed of the actuator against the operator's intention.